1. Field of the Invention
The present invention relates to a rechargeable battery and, more particularly, to a rechargeable battery with an improved structure of a cap assembly.
2. Description of the Related Art
Unlike a primary battery that cannot be recharged, a rechargeable battery (i.e., a secondary battery or a secondary cell) can be repeatedly charged and discharged. A low-capacity rechargeable battery including a single cell is used for small portable electronic devices such as mobile phones, notebook computers, camcorders, and the like. Meanwhile, a large-capacity rechargeable battery in which a plurality of cells are connected in the form of a pack is commonly used as a power source for driving a motor of a hybrid electric vehicle and the like.
Such rechargeable batteries are manufactured to have various shapes, and typical shapes include cylindrical or angular shapes.
The rechargeable batteries may be connected in series to constitute a large-capacity rechargeable battery module so as to be used for driving a motor of electric vehicles and the like that require more power.
The rechargeable battery includes an electrode group (i.e., an electrode assembly) having a positive electrode and a negative electrode positioned with a separator interposed therebetween, a case having a space in which the electrode group is installed, and a cap assembly for sealing the case.
When the rechargeable battery has a cylindrical shape, the positive electrode and negative electrode of the electrode group include an uncoated region without an active material which are disposed in different directions.
A negative electrode current collecting plate is attached to the negative electrode uncoated region, and the positive electrode current collecting plate is attached to the positive electrode uncoated region. The negative electrode current collecting plate is electrically connected to the case, and the positive electrode current collecting plate is electrically connected with the cap assembly to externally induce current. Accordingly, the case serves as a negative electrode terminal, and a cap-up installed at the cap assembly serves as an positive electrode terminal.
When the rechargeable battery is repeatedly charged and discharged, a gas is generated therein, increasing its internal pressure. If the increased pressure within the rechargeable battery is allowed to remain, there is a high possibility that the rechargeable battery may explode. Thus, in order to avoid that, a vent plate with a notch is installed below the cap-up such that it is broken at a certain pressure thereby releasing the excess pressure.
Typically, the cap-up is made of steel with good rigidity, and is formed to be thick to maintain a certain strength. The vent plate is typically formed of an aluminum plate that is relatively thin so as to be easily broken at a pre-set pressure. The cap-up and the vent plate are joined (bonded) through welding, but in this case, the cap-up and the vent plate are not properly welded because their melting temperatures are different.
Namely, if the welding temperature is increased to melt the cap-up, the vent plate can excessively melt to thereby damage the welding portion, and if the welding temperature is low, the cap-up is not properly melted which weakens the strength of the welded portion.
When such defective welding occurs, high resistance is generated from the welding portion which may degrade output, and the welding portion may be more easily broken due to an external impact or the like which can cause the rechargeable battery to fail to transmit current.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.